This invention relates generally to the preparation of metallic microstructures. More specifically, the invention relates to the fabrication of metallic components of micron or submicron dimensions using compositions containing metallic nanoparticles and lithographically or otherwise patterned molds. The invention pertains to miniaturization and xe2x80x9cnanotechnology,xe2x80x9d and has utility in many fields, including microelectromechanical system fabrication, semiconductor processing, information storage, medical diagnostics, optics, materials science, and structural engineering.
xe2x80x9cNanotechnologyxe2x80x9d refers to nanometer-scale manufacturing processes, materials and devices, as associated with, for example, nanometer-scale lithography and nanometer-scale information storage. See, for example, Nanotechnology, ed. G. Timp (New York: Springer-Verlag, 1999), and Nanoparticles and Nanostructured Films, ed. J. H. Fendler (Weinheim, Germany: Wiley-VCH, 1998). Nanometer-scale components find utility in a wide variety of fields, particularly in the fabrication of microelectromechanical systems (commonly referred to as xe2x80x9cMEMSxe2x80x9d). Such systems include, for example, micro-sensors, micro-actuators, micro-instruments, micro-optics, and the like. Many MEMS fabrication processes exist and they tend to fall into the two categories of surface micro-machining and bulk-micromachining. The latter technique involves formation of microstructuring by etching directly into a bulk material, typically using wet chemical etching or reactive ion etching (xe2x80x9cRIExe2x80x9d). Surface micro-machining involves fabrication of microelectromechanical systems from films deposited on the surface of a substrate, e.g., from thin layers of polysilicon deposited on a sacrificial layer of silicon dioxide present on a single crystal silicon substrate (this technique is commonly referred to as the xe2x80x9cthin film polysilicon processxe2x80x9d).
An exemplary surface micro-machining process is known as xe2x80x9cLIGA.xe2x80x9d See, for example, Becker et al. (1986), xe2x80x9cFabrication of Microstructures with High Aspect Ratios and Great Structural Heights by Synchrotron Radiation Lithography Galvanoforming, and Plastic Moulding (LIGA Process),xe2x80x9d Microelectronic Engineering 4(1):35-36; Ehrfeld et al. (1988), xe2x80x9c1988 LIGA Process: Sensor Construction Techniques via x-Ray Lithography,xe2x80x9d Tech. Digest from IEEE Solid-State Sensor and Actuator Workshop, Hilton Head, S.C.; Guckel et al. (1991) J. Micromech. Microeng. 1: 135-138. A related process is termed xe2x80x9cSLIGA,xe2x80x9d and refers to a LIGA process involving sacrificial layers. LIGA is the German acronym for X-ray lithography (xe2x80x9clithographicxe2x80x9d), electrodeposition (xe2x80x9cgalvanoformungxe2x80x9d) and molding (xe2x80x9cabformungxe2x80x9d), and was developed in the mid-1970""s. LIGA involves deposition of a relatively thick layer of an X-ray resist on a substrate, e.g., metallized silicon, followed by exposure to high-energy X-ray radiation through an X-ray mask, and removal of the irradiated resist portions using a chemical developer. The mold so provided can be used to prepare structures having horizontal dimensions, i.e., diameters, on the order of microns. The technique is now used to prepare metallic microcomponents by electroplating in the recesses (i.e., the developed regions) of the LIGA mold. See, for example, U.S. Pat. Nos. 5,190,637 and 5,576,147 both to Guckel et al.
While metallic microcomponents are useful in a host of applications, currently microcomponents can only be fabricated from electroplateable metals. The ability to produce metallic components from non-electroplateable metals is obviously desirable as well. Microcomponents produced from such non-electroplateable metals, i.e., stainless steel, aluminum, titanium, and shape memory alloy, would clearly be useful in a number of applications, insofar as such materials can provide a host of advantageous properties, including increased toughness, thermal stability, chemical and biological compatibility, magnetism, etc. To date, however, no suitable method has been developed for the fabrication of metallic microstructures from non-electroplateable materials or for the fabrication of metallic microstructures comprised of uniform alloy compositions.
Accordingly, the invention is directed to the aforementioned need in the art and provides a method for making metallic microstructures, i.e., metallic components of micron or submicron dimensions from virtually any metal or metal alloy, electroplateable or not.
It is another object of the invention to provide a method that involves compressing, into a patterned mold, a molding composition comprising metallic nanoparticles and an optional binder, and then removing and at least partially sintering the resulting metallic microstructures.
It is still another object of the invention to provide such a method wherein the patterned mold is a lithographically patterned mold such as a LIGA mold.
It is still another object of the invention to provide such a method wherein the patterned mold is an epoxy-based negative resist patterned mold such as SU8-25.
It is still another object of the invention to provide such a method wherein the patterned mold is a plastic replicate of a master mold prepared via LIGA or other technologies. Such replicates might be fabricated by hot embossing or injection molding technologies.
It is yet another object of the invention to provide such a method wherein the molding composition additionally comprises a fluxing agent.
It is still a further object of the invention to provide novel metallic microcomponents fabricated using the methodology disclosed and claimed herein.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and, in part, will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In one aspect of the invention, then, a process for preparing metallic microstructures is provided which involves compression molding a molding composition in a suitable mold (typically a lithographically patterned mold such as a LIGA mold or a negative photoresist or a plastic replicate thereof) wherein the molding composition is comprised of nanoparticles of a metallic material and an optional binder. The compressed molding composition provides green metallic microstructures within the voids of the patterned relief surface on the mold that is employed. Following planarization, the green microstructures are removed from the mold and are at least partially sintered. Metallic components of micron or submicron dimensions can be prepared in this manner with any metallic material that is capable of being provided in a nanoparticulate form. In addition, depending on the metallic material selected, metallic microstructures can be fabricated with desirable optical, structural, magnetic, or other properties. Partial sintering allows for the formation of nanoporous microstructures, which may be used in filtration and/or separation.
In an additional aspect of the invention, certain metallic microstructures are prepared as novel compositions of matter. The novel metallic microstructures comprise a sintered, or partially sintered, compressed solid of metallic nanoparticles.